The efficient conversion of solar energy into electricity has been a major goal of the global search for renewable energy sources. Considerable progress has been made in this search with the development of photovoltaic devices based on silicon (Si). The Si solar cell depends on the fabrication of p-n junctions usually produced by doping Si with B (p-type) and with P (n-type). Single crystal Si, polycrystalline Si, or amorphous hydrogenated Si can be employed. With the latter material, thin film ˜1 μm thick cells can be constructed because of the high absorption coefficient of amorphous Si for solar radiation compared to crystalline Si. Solar cells based on Si have a theoretical conversion efficiency of 22%. However, various loss factors prevent one from reaching the theoretical efficiency and thin film cells generally have conversion efficiencies of 5-8%. One of the principal reasons for the low conversion efficiency is the fact that the band gap of Si is ˜1 ev. Solar photons with energies less than 1 ev are not effective in generating an electrical current. On the other hand, photons with energies higher than 1 ev appear to generate no more than one electron-hole pair so that much of the excess energy is degraded to lattice vibrational energy and appears as heat. To improve cell performance, composite cells have been developed consisting of several materials stacked one on top of the other. The variation in the band gaps of these materials allows selective absorption of photons of different energies to occur so that the overall efficiency of such a composite cell is increased. Fabrication of such composite cells of course increases the cost of the devices considerably.
An intensive search has been made during the last twenty years for materials that would allow one to increase the photon to electricity conversion efficiency without adding substantially to the cost of the photovoltaic cells. A material for this purpose ideally would have the property of making optimal use of photons with energies spanning the solar spectrum on the surface of the earth, that is to say photon energies in the range of 0.5-5 ev. To make use of the solar photons, the material must have a high absorption coefficient over the entire solar spectral range. The material should have a wide band gap of 5-6 ev, be electrically conducting, and be capable of being doped both n and p type. Furthermore, the electronic structure and the electron-photon coupling must be such as to allow the creation of electron-hole pairs which are effective in sustaining an electric current with good quantum efficiency over the whole solar spectral range. Considering the stringent materials requirements which have just been enumerated, it is not surprising that no material has been found up to now which satisfies all of the above named criteria.
The present invention is a new photovoltaic device based on carbon in the form of diamond which can function as a high efficiency solar cell and a method of making same. The discovery, development characterization and uses of ultrananocrystalline diamond films (UNCD) that is diamond having average diameters in the range of from about 3 nanometers to about 15 nanometers has been the subject of a number of earlier patents, such as U.S. Pat. No. 5,989,511, and Ser. No. 10/398,329 filed Apr. 4, 2003, the disclosures of which are herein incorporated by reference, teach the nitrogen doping of UNCD films to enhance the electrical conductivity and therefore the electron emission and electrochemical properties of UNCD. The nature of the carriers giving rise to the conductivity has been found to be n-type by Hall effect measurements.